Effects of thoracic volume and shape on electromechanical coupling in abdominal muscles

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Abstract
To assess the effects of lung volume and chest wall configuration on electromechanical coupling of the abdominal muscles, we examined the relationship between abdominal muscle pressure ( Pmus ) and electrical activity ( EMGab ) in eight normal subjects during expiratory efforts at lung volumes ranging from functional residual capacity (FRC) to FRC + 2.0 liters. At and above FRC, increases of lung volume did not significantly alter either the Pmus - EMGab relationship or abdominal surface linear dimensions, although expiratory efforts displaced the abdomen inward from its relaxed position. We attribute the constancy of delta Pmus /delta EMG above FRC to the negligible effects of increasing lung volume on abdominal configuration and muscle length. Expiratory efforts performed at lung volumes below FRC resulted in a wider range of abdominal indrawing . Under these conditions the EMGab required to augment Pmus by 30–40 cmH2O increased as the abdomen was displaced inward. This decrease of delta Pmus /delta EMGab appears to reflect muscle shortening, flattening of the abdominal wall, and possibly deformation of the rib cage.